Artifacts when baking with multires
It looks like you try to make your mesh look smooth by adding more mesh subdivisions instead of setting its shading to smooth.

Unless you're on an insane project and you have all the power of the world to be able to spend hundreds of polygons on the slightest prop, this isn't the right way.
Adding mesh should be needed only if you need those extra polygons to form volume. The smooth appearance should be before all managed through shading.

Since normal baking calculates your faces' normal as is, if you're using flat shading, the baked normals will be flat as well, hence this weird patterns.

See this Blender 2.79 manual page to learn more about normals.

In short, to fix this:


In the 3D Viewport, select your mesh
In the tools panel (toggle it with T), click the Smooth button in the Edit panel:




Then your baking should be correct.